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Astron. Astrophys. 351, 1075-1086 (1999)

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10. Conclusions

The spectra of several positions inside seven Galactic HII regions have been used to obtain an homogeneous set of data comprising temperatures ([FORMULA] and [FORMULA]) densities ([FORMULA] and [FORMULA]) and the abundances of O+, O++, S+, S++, Cl +, Cl ++, N+, Ar ++, C++ and He + . These data have been used to study the trends followed by temperatures, densities and the ionization fractions of different elements. The relative behaviour of the calculated ionic abundances by itself, without any assumptions concerning the ionization correction factors, implies that the chemical abundances are similar in all the HII regions studied. The preferred values for the abundances, within [FORMULA] dex, are: [FORMULA], [FORMULA], [FORMULA], [FORMULA] and [FORMULA], whereas [FORMULA]. All these abundance ratios are below the solar values, but are similar to the abundances derived in B stars and the interstellar medium.

The main result of this work is that - regardless of the real values of the abundances, which can be affected by errors in the temperatures, the atomic data or the method used - all the HII regions studied are characterized by similar abundances. Whenever there are discrepancies, these can be attributed to uncertainties related to the temperature or to a low signal-to-noise in the corresponding spectrum. Taking into account that all the objects considered have galactocentric distances in the range 6-10 kpc, this result does not preclude the existence of a Galactic abundance gradient: it has even been suggested (Simpson et al. 1995) that the galactocentric variations of [FORMULA], [FORMULA] and [FORMULA] in HII regions with galactocentric distances below 10 kpc can be better explained by a step fit with two levels (such that [FORMULA] and [FORMULA] from 6 to 10 kpc) than with the usual fit to a linear gradient. The presence of a flat gradient over this range of galactocentric distances is also suggested by several studies on the abundances of O in B-type stars (e.g. Smartt & Rolleston 1997), although its reality is still a matter of debate (e.g. Gummersbach et al. 1998). The results presented here are consistent with the flattening or mild variation of the Galactic abundance gradient in the range 6-10 kpc for all the elements whose total abundances have been estimated or constrained: He, O, S, Cl, N and Ar .

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© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999